Surface preparation prior to deposition on germanium

ABSTRACT

Methods are provided for treating germanium surfaces in preparation for subsequent deposition, particularly gate dielectric deposition by atomic layer deposition (ALD). Prior to depositing, the germanium surface is treated with plasma products or thermally reacted with vapor reactants. Examples of surface treatments leave oxygen bridges, nitrogen bridges, —OH, —NH and/or —NH 2  terminations that more readily adsorb ALD reactants. The surface treatments avoid deep penetration of the reactants into the germanium bulk but improve nucleation.

PRIORITY APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication 60/492,408, filed 4 Aug. 2003, the entire disclosure ofwhich is hereby incorporated by reference herein.

FIELD OF THE INVENTION

The invention relates generally to surface preparation prior to filmdeposition for semiconductor fabrication, and more particularly togermanium surface preparation to facilitate nucleation in subsequentvapor deposition processes.

BACKGROUND OF THE INVENTION

Integrated circuit design is constantly being scaled down in pursuit offaster circuit operation and lower power consumption. Scaled dimensionsin a circuit design generally requires attendant changes in fabricationprocessing.

A basic building block of integrated circuits is the thin filmtransistor (TFT). As is known in the art, the transistor typicallyincludes a gate electrode separated from a semiconductor layer orsubstrate by a thin gate dielectric material. One area in which processcontrol is particularly critical is the fabrication of transistor gatedielectrics. In the pursuit of ever faster and more efficient circuits,semiconductor designs are continually scaled down with each productgeneration. Transistor switching time plays a large role in the pursuitof faster circuit operation. Switching time, in turn, can be reduced byreducing the channel length of the transistors. In order to realizemaximum improvements in transistor performance, vertical dimensionsshould be scaled along with horizontal dimensions. Accordingly,effective gate dielectric thickness, junction depth, etc. will alldecrease with future generation integrated circuits.

Conventional gate dielectrics are formed of high quality silicon dioxideand are typically referred to as “gate oxide” layers. Ultra thin gateoxides (e.g., less than 5 nm), however, have been found to exhibit highdefect densities, including pinholes, charge trapping states, andsusceptibility to hot carrier injection effects. Such high defectdensities lead to leakage currents through the gate dielectric and rapiddevice breakdown unacceptable for circuit designs with less than 0.25 μmgate spacing, i.e., sub-quarter-micron technology. Even if the integrityof the oxide is perfectly maintained, quantum-mechanical effects setfundamental limits on the scaling of gate oxide due to quantum tunnelingeffects.

Theoretically, incorporating materials of higher dielectric constantinto the gate dielectric opens the door to further device scaling. Dueto higher dielectric constant, many materials can exhibit the samecapacitance as a thinner silicon dioxide layer, such that a lowerequivalent oxide thickness (EOT) can be achieved without tunnel-limitedbehavior. Silicon nitride (Si₃N₄), for example, has a higher dielectricconstant (“k value”) than SiO₂ and also demonstrates good diffusionbarrier properties, resisting boron penetration. More exotic “high k”materials with even higher dielectric constants, including aluminumoxide (Al₂O₃), zirconium oxide (ZrO₂), hafnium oxide (HfO₂), bariumstrontium titanate (BST), strontium bismuth tantalate (SBT), tantalumoxide (Ta₂O₅), etc., are also being investigated to allow further devicescaling.

Recent developments on another front in transistor design have focusedon increasing the electrical carrier mobility in the single-crystalsemiconductor material (substrate or epitaxial layer) in which thetransistors are formed. One method of increasing carrier mobility is toproduce strained layers, such as strained silicon over relaxed silicongermanium.

Pure germanium also exhibits greater carrier mobility than silicon,whether or not the germanium crystal structure is strained. Untilrecently, interest in germanium has been limited, in part, by theinability to grow consistent, high quality oxides from the surface, suchthat gate dielectrics would have to be deposited. Since oxides thermallyor chemically grown from silicon exhibited the highest quality and mostconsistent thickness, a preference for silicon dioxide gate dielectricshas until recently dictated a preference for silicon over germanium asthe semiconductor material of choice.

The growing preference for deposited high k dielectrics, rather thangrown oxides, now has the potential to obviate the most significantdisadvantage to germanium as the semiconductor base layer fortransistor. The possibility for high carrier mobility in germaniumcombined with scaled dimensions enabled by high k dielectrics is veryattractive for future integrated circuit design.

Despite significant advantages for these materials, both germanium andhigh k materials raise their own integration challenges. Accordingly,significant advances are required before production worthy methods areavailable for integrating these new materials into fabrication processflows.

SUMMARY OF THE INVENTION

Uniformity in dielectric thickness is particularly important for deviceconsistency. Whereas silicon dioxide could be easily chemically orthermally grown from silicon at a uniform thickness, depositiontechniques tend not to produce such uniformity.

One method of reliable, almost perfectly conformal deposition ofextremely thin layers is atomic layer deposition (ALD). This cyclicaldeposition technique has the advantage of self-limiting surfacereactions, such that thickness uniformity does not depend upon uniformsubstrate temperatures (in contrast to CVD in the kinetic regime) norupon uniform reactant supplies (in contrast to PVD and to CVD in themass transport regime).

Despite these advantages, the inventor has recognized limitations uponthe uniformity offered by ALD. In particular, ALD is prone to nucleationeffects, depending upon the reactants and surfaces upon which depositionis to start. At the thickness ranges envisioned for high k gatedielectrics, thickness non-uniformities due to inconsistent nucleationcan critically affect device performance. Furthermore, the inventor hasrecognized that germanium surfaces are especially subject to poornucleation of typical ALD reactions.

Accordingly, a need exists for improving the speed, efficiency, qualityand uniformity of depositing layers upon germanium surfaces. Insatisfaction of this need, methods are provided herein for treatinggermanium surfaces in preparation for subsequent deposition.

In accordance with one aspect of the invention, a germanium surface istreated with plasma products prior to deposition thereover. Inaccordance with another aspect of the invention, a germanium surface isthermally reacted with vapor phase reactants prior to depositionthereover. Exemplary surface treatments include provision of oxygen-and/or nitrogen-containing vapor reactants, activated thermally orthrough a remote plasma module attached to a deposition chamber, for insitu surface treatment prior to deposition.

Preferred embodiments leave one or a mixture of oxygen bridges, nitrogenbridges, —OH, —NH and —NH₂ surface groups at the germanium surface priorto atomic layer deposition of a high k dielectric. Advantageously, ALDreactants more readily adsorb upon the treated surface. By changing thesurface termination of the substrate with a low temperature surfacetreatment, subsequent deposition is advantageously facilitated withoutsignificantly affecting the bulk properties of the underlying material.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be readily apparent fromthe following description and from the appended drawings, which aremeant to illustrate and not to limit the invention, and wherein:

FIG. 1 is a schematic sectional view of an exemplary single-substratereaction chamber;

FIG. 2 is a gas flow schematic, illustrating reactant and purge gassources in accordance with a preferred embodiment of the presentinvention;

FIG. 3 is a flow chart generally showing steps for treating substratesin accordance with the preferred embodiments; and

FIGS. 4A and 4B are schematic sections of a transistor gate stackconstructed in accordance with preferred embodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Treatments are described herein for preparing germanium surfaces forsubsequent deposition. The thermal reaction or plasma productspreferably modify termination of the germanium surface to make it moreuniformly susceptible to subsequent deposition. The treatmentspreferably provide a consistent density of nucleation sites across thesurface. While the treatment processes are described herein inconjunction with adsorption-driven reactions of atomic layer depositionover the treated germanium surface, the skilled artisan will readilyappreciate that the methods taught herein will also be applicable tonucleation of other deposition processes for a variety of materials overgermanium surfaces.

Atomic layer deposition (ALD) is a self-limiting process, wherebyalternated pulses of reaction precursors saturate a substrate and leaveno more than one monolayer of material per pulse. The precursors areselected to ensure self-saturating reactions, because an adsorbed layerin one pulse leaves a surface termination that is non-reactive with thegas phase reactants of the same pulse. A subsequent pulse of differentreactants does react with the previous termination to enable continueddeposition. Thus, each cycle of alternated pulses leaves no more thanabout one molecular layer of the desired material. The principles of ALDtype processes have been presented by T. Suntola, e.g. in the Handbookof Crystal Growth 3, Thin Films and Epitaxy, Part B: Growth Mechanismsand Dynamics, Chapter 14, Atomic Layer Epitaxy, pp. 601–663, ElsevierScience B.V. 1994, the disclosures of which are incorporated herein byreference.

Unfortunately, depending upon the chemistries employed, ALD does notdeposit equally well on different starting substrates. Some ALD processrecipes, for example, have been found slow or even non-operative indepositing over silicon, and particularly etched or cleaned siliconsurfaces (typically hydrogen-terminated). For example, it is unlikelythat aluminum alkyls, such as (CH₃)₃Al, can attach on ahydrogen-terminated silicon surface in ALD processes for depositingAl₂O₃.

Germanium has been found similarly inhospitable to ALD reactions.Cleaned germanium surfaces provide poor adsorption sites for typical ALDreactants under typical ALD conditions, which include temperatures lowenough to avoid thermal decomposition of the reactants and high enoughto avoid condensation. Germanium does not readily oxidize, moreover,such that its native oxide is of a particularly poor quality andconsistency. The spotty and unstable oxide results in very slow andrough deposition by many CVD and ALD process recipes.

Intermediate layers are often deposited prior to deposition of thedesired functional layer for a variety of remedial reasons, includingotherwise poor adhesion, nucleation, electrical interface properties,diffusion, etc. Such intermediate layers add to the complexity and costof fabrication, and can also occupy valuable space within high aspectratio features, such as trenches in germanium substrates. For gatedielectrics, additional layers increase the overall dielectric thicknessand reduce the effectiveness of the layer, contrary to the trend forscaling down integrated circuits.

Surface preparation can be conducted by wet bench treatments. See, e.g.,Bai et al., “Ge MOS Characteristics with CVD HfO₂ Gate Dielectrics andTaN Gate Electrode,” 2003 Symp. on VLSI Tech. Digest of Tech. Papers(June 2003). However, such wet chemical treatments are prone to impurityissues, both in the treatment baths and in transit from such baths todeposition tools. Accordingly, the preferred processes described hereinemploy vapor processes, and particularly in situ processing within thetool in which deposition over germanium surfaces is to be conducted.

Preferred Reactor

Prior to describing the processes in greater detail, an exemplaryreactor for surface treatments and vapor deposition processes is firstdescribed below. While not illustrated separately, the surface treatmentand ALD processes described below can be performed in a Pulsar™ 3000reactor, commercially available from ASM America, Inc. of Phoenix,Ariz., for certain embodiments with a remote plasma processing unitconnected thereto.

The preferred embodiments are presented in the context of asingle-substrate, horizontal flow cold-wall reactor. The illustratedsingle-pass horizontal flow design enables laminar flow of reactantgases, with low residence times, which in turn facilitates sequentialprocessing while minimizing reactant interaction with each other andwith chamber surfaces. Thus, among other advantages, such a laminar flowenables sequentially flowing reactants that might react with each other.Reactions to be avoided include highly exothermic or explosivereactions, such as produced by oxygen and hydrogen-bearing reactants,and reactions that produce particulate contamination of the chamber.

The skilled artisan will recognize, however, that other reactor designscan also be provided for achieving these ends. For example, theprocesses described herein need not be performed in situ within a singlechamber. If the surface treatments are performed in a different chamberfrom the depositions, the chambers are preferably clustered togetheraround a high purity transfer chamber. Moreover, the processes describedherein can be readily implemented in batch processing tools, singlewafer tools that employ showerhead arrangements, etc.

FIG. 1 shows the exemplary vapor deposition reactor 10, including aquartz process or reaction chamber 12, constructed in accordance with apreferred embodiment, and for which the methods disclosed herein haveparticular utility. While originally designed to optimize epitaxialdeposition of silicon on a single substrate at a time, the inventorshave found the superior processing control to have utility in of anumber of different processes. Moreover, the illustrated reactor 10 cansafely and cleanly accomplish multiple treatment steps sequentially inthe same chamber 12. The basic configuration of the reactor 10 isavailable commercially under the trade name Epsilon® from ASM America,Inc. of Phoenix, Ariz.

A plurality of radiant heat sources are supported outside the chamber 12to provide heat energy in the chamber 12 without appreciable absorptionby the quartz chamber 12 walls. While the preferred embodiments aredescribed in the context of a “cold wall” CVD reactor for processingsemiconductor wafers, it will be understood that the processing methodsdescribed herein will have utility in conjunction with otherheating/cooling systems, such as those employing inductive or resistiveheating.

The illustrated radiant heat sources comprise an upper heating assemblyof elongated tube-type radiant heating elements 13. The upper heatingelements 13 are preferably disposed in spaced-apart parallelrelationship and also substantially parallel with the reactant gas flowpath through the underlying reaction chamber 12. A lower heatingassembly comprises similar elongated tube-type radiant heating elements14 below the reaction chamber 12, preferably oriented transverse to theupper heating elements 13. Desirably, a portion of the radiant heat isdiffusely reflected into the chamber 12 by rough specular reflectorplates (not shown) above and below the upper and lower lamps 13, 14,respectively. Additionally, a plurality of spot lamps 15 supplyconcentrated heat to the underside of the substrate support structure(described below), to counteract a heat sink effect created by coldsupport structures extending through the bottom of the reaction chamber12.

Each of the elongated tube type heating elements 13, 14 is preferably ahigh intensity tungsten filament lamp having a transparent quartzenvelope containing a halogen gas, such as iodine. Such lamps producefull-spectrum radiant heat energy transmitted through the walls of thereaction chamber 12 without appreciable absorption. As is known in theart of semiconductor processing equipment, the power of the variouslamps 13, 14, 15 can be controlled independently or in grouped zones inresponse to temperature sensors.

A substrate, preferably comprising a silicon wafer 16, is shownsupported within the reaction chamber 12 upon a substrate supportstructure 18. Note that, while the substrate of the illustratedembodiment is a single-crystal silicon wafer, it will be understood thatthe term “substrate” broadly refers to any surface on which a layer isto be deposited. Moreover, the principles and advantages describedherein apply equally well to depositing layers over numerous other typesof substrates, including, without limitation, glass substrates such asthose employed in flat panel displays.

The illustrated support structure 18 includes a substrate holder 20,upon which the wafer 16 rests, and a support spider 22. The spider 22 ismounted to a shaft 24, which extends downwardly through a tube 26depending from the chamber lower wall. Preferably, the tube 26communicates with a source of purge or sweep gas which can flow duringprocessing, inhibiting process gases from escaping to the lower sectionof the chamber 12.

A plurality of temperature sensors are positioned in proximity to thewafer 16. The temperature sensors may take any of a variety of forms,such as optical pyrometers or thermocouples. The number and positions ofthe temperature sensors are selected to promote temperature uniformity,as will be understood in light of the description below of the preferredtemperature controller. Preferably, however, the temperature sensorsdirectly or indirectly sense the temperature of positions in proximityto the wafer.

In the illustrated embodiment, the temperature sensors comprisethermocouples, including a first or central thermocouple 28, suspendedbelow the wafer holder 20 in any suitable fashion. The illustratedcentral thermocouple 28 passes through the spider 22 in proximity to thewafer holder 20. The reactor 10 further includes a plurality ofsecondary or peripheral thermocouples, also in proximity to the wafer16, including a leading edge or front thermocouple 29, a trailing edgeor rear thermocouple 30, and one or more side thermocouples (not shown).Each of the peripheral thermocouples are housed within a slip ring 32,which surrounds the substrate holder 20 and the wafer 16. Each of thecentral and peripheral thermocouples are connected to a PID temperaturecontroller, which sets the power of the various heating elements 13, 14,15 in response to the readings of the thermocouples.

In addition to housing the peripheral thermocouples, the slip ring 32absorbs and emits radiant heat during high temperature processing, suchthat it compensates for a tendency toward greater heat loss orabsorption at wafer edges, a phenomenon which is known to occur due to agreater ratio of surface area to volume in regions near such edges. Byminimizing edge losses, the slip ring 32 can reduce the risk of radialtemperature non-uniformities across the wafer 16. The slip ring 32 canbe suspended by any suitable means. For example, the illustrated slipring 32 rests upon elbows 34 which depend from a front chamber divider36 and a rear chamber divider 38. The dividers 36, 38 desirably areformed of quartz. In some arrangements, the rear divider 38 can beomitted.

The illustrated reaction chamber 12 includes an inlet port 40 for theinjection of reactant and carrier gases, and the wafer 16 can also bereceived therethrough. An outlet port 42 is on the opposite side of thechamber 12, with the wafer support structure 18 positioned between theinlet 40 and the outlet 42.

An inlet component 50 is fitted to the reaction chamber 12, adapted tosurround the inlet port 40, and includes a horizontally elongated slot52 through which the wafer 16 can be inserted. A generally verticalinlet 54 receives gases from remote sources, as will be described morefully with respect to FIG. 2, and communicates such gases with the slot52 and the inlet port 40. The inlet 54 can include gas injectors asdescribed in U.S. Pat. No. 5,221,556, issued Hawkins et al., or asdescribed with respect to FIGS. 21–26 in U.S. Pat. No. 6,093,252, issuedJul. 25, 2000, the disclosures of which are hereby incorporated byreference. Such injectors are designed to maximize uniformity of gasflow for the single-wafer reactor.

An outlet component 56 similarly mounts to the process chamber 12 suchthat an exhaust opening 58 aligns with the outlet port 42 and leads toexhaust conduits 59. The conduits 59, in turn, can communicate withsuitable vacuum means (not shown) for drawing process gases through thechamber 12. In the preferred embodiment, process gases are drawn throughthe reaction chamber 12 and a downstream scrubber (not shown). A pump orfan is preferably included to aid in drawing process gases through thechamber 12, and to evacuate the chamber for low pressure processing.

The preferred reactor 10 also includes a source 60 of excited species,preferably positioned upstream from the chamber 10. The excited speciessource 60 of the illustrated embodiment comprises a remote plasmagenerator, including a magnetron power generator and an applicator alonga gas line 62. An exemplary remote plasma generator is availablecommercially under the trade name TRW-850 from Rapid Reactive RadicalsTechnology (R3T) GmbH of Munich, Germany. In the illustrated embodiment,microwave energy from a magnetron is coupled to a flowing gas in anapplicator along a gas line 62. A source of precursor gases 63 iscoupled to the gas line 62 for introduction into the excited speciesgenerator 60. A source of carrier gas 64 is also coupled to the gas line62. One or more further branch lines 65 can also be provided foradditional reactants. As is known in the art, the gas sources 63, 64 cancomprise gas tanks, bubblers, etc., depending upon the form andvolatility of the reactant species. Each gas line can be provided with aseparate mass flow controller (MFC) and valves, as shown, to allowselection of relative amounts of carrier and reactant species introducedto the excited species generator 60 and thence into the reaction chamber12.

It will be understood that, in other arrangements, the excited speciescan be generated within the process chamber. For example, in situplasmas can be generated by applying radio frequency (RF) power tospaced electrodes within the process chamber, as is known in the art.Exemplary in situ plasma CVD reactors are available, for example, fromASM Japan K.K. of Tokyo, Japan under the trade name Eagle™ 10 or Eagle™12. Furthermore, energy can be coupled to source gases by a number ofmeans, including by induction, capacitively, etc., for either in situ orremote plasma generation. Preferably, however, a remote plasma source isemployed for the processes described herein, affording greater controlfor surface modification with minimal bulk effects.

Wafers are preferably passed from a handling chamber (not shown), whichis isolated from the surrounding environment, through the slot 52 by apick-up device. The handling chamber and the processing chamber 12 arepreferably separated by a gate valve (not shown) of the type disclosedin U.S. Pat. No. 4,828,224, the disclosure of which is herebyincorporated herein by reference.

The total volume capacity of a single-wafer process chamber 12 designedfor processing 200 mm wafers, for example, is preferably less than about30 liters, more preferably less than about 20 liters, and mostpreferably less than about 10 liters. The illustrated chamber 12 has acapacity of about 7.5 liters. Because the illustrated chamber 12 isdivided by the dividers 32, 38, wafer holder 20, ring 32, and the purgegas flowing from the tube 26, however, the effective volume throughwhich process gases flow is around half the total volume (about 3.77liters in the illustrated embodiment). Of course, it will be understoodthat the volume of the single-wafer process chamber 12 can be different,depending upon the size of the wafers for which the chamber 12 isdesigned to accommodate. For example, a single-wafer processing chamber12 of the illustrated type, but for 300 mm wafers, preferably has acapacity of less than about 100 liters, more preferably less than about60 liters, and most preferably less than about 30 liters. One 300-mmwafer processing chamber has a total volume of about 24 liters, with aneffective processing gas capacity of about 11.83 liters.

FIG. 2 shows a gas line schematic, in accordance with the preferredembodiment. The reactor 10 is provided with a thermal reactant source 70of oxygen and/or nitrogen. The thermal reactant source 70 can compriseany of a number of known oxygen-and/or nitrogen-containing chemicals,particularly a volatile chemical such as O₂, O₃, H₂O, H₂O₂, NO, N₂O,N₂O, N₂, N₂/H₂, HCOOH, HClO₃, CO₂, mixtures of the fore Preferably, thereactant is introduced in an inert carrier gas flow, such as N₂ or noblegas, although pure reactant flows can also be used. Alternatively or inaddition, an oxygen-containing and/or nitrogen gas source 63 can beconnected to the remote plasma generator 60 to provide excited speciesfor surface treatment.

As also shown in FIG. 2, the reactor 10 further includes a source 72 ofhydrogen gas (H₂). As is known in the art, hydrogen is a useful carriergas and purge gas because it can be provided in very high purity, due toits low boiling point, and is compatible with silicon deposition. H₂ canalso be employed in a high temperature hydrogen bake to sublimate nativeoxide prior to layer formation. H₂ can also flow through the excitedspecies generator 60 to generate H radicals for native oxide cleaning orfor other purposes.

The preferred reactor 10 also includes a source 73 of nitrogen gas (N₂).As is known in the art, N₂ is often employed in place of H₂ as a carrieror purge gas in semiconductor fabrication. Nitrogen gas is relativelyinert and compatible with many integrated materials and process flows.Other possible carrier gases include noble gases, such as helium (He) orargon (Ar).

A liquid reactant source 74 is also shown. The bubbler can hold liquidorganometallic precursors, such as Ta(OC₂H₅)₅, while a gas line servesto bubble H₂, N₂, Ne, He or Ar through the liquid metal source andtransport metallorganic precursors to the reaction chamber 12 in gaseousform.

When semiconductor deposition (e.g., Si, Ge, SiGe) is also performed inthe same chamber, the liquid source 74 can comprise, for example, liquiddichlorosilane (DCS), trichlorosilane (TCS), trisilane or other highorder silane sources in a bubbler. In such cases, or when gaseoussemiconductor sources such as the illustrated silane source 86 ortrisilane source are used, the reactor 10 can also include other sourcegases such as dopant sources (e.g., the illustrated phosphine 76, arsine78 and diborane 80 sources) and etchants for cleaning the reactor wallsand other internal components (e.g., HCl source 82 or NF₃/Cl₂ providedas the plasma source gas 63 for feeding the excited species generator60). For deposition of germanium-containing materials (e.g., thegermanium base layer or SiGe layers), a source of germanium 84 (e.g.,germane or GeH₄ as shown) can also be provided.

Each of the gas sources may be connected to the inlet 54 (FIG. 1) viagas lines with attendant safety and control valves, as well as mass flowcontrollers (“MFCs”), which are coordinated at a gas panel. Processgases are communicated to the inlet 54 (FIG. 1) in accordance withdirections programmed into a central controller and distributed into theprocess chamber 12 through injectors. After passing through the processchamber 12, unreacted process gases and gaseous reaction by-products areexhausted to a scrubber 88 to condense environmentally dangerous flumesbefore exhausting to the atmosphere.

In addition to the conventional gas sources and liquid bubblers,discussed above, the preferred reactor 10 includes the excited speciessource 60 positioned remotely or upstream of the reaction chamber 12.The illustrated source 60 couples microwave energy to gas flowing in anapplicator, where the gas includes reactant precursors from the reactantsource 63. For the processes described below, the plasma source gases 63include a source of oxygen and/or a source of nitrogen. Other usefulplasma source gases for the preferred processes include N₂, and noblegases as plasma-supporting carrier gases. A plasma is ignited within theapplicator, and excited species are carried toward the chamber 12.Preferably, of the excited species generated by the source 60, overlyreactive ionic species substantially recombine prior to entry into thechamber 12. On the other hand, radicals such as N or O survive to enterthe chamber 12 and react as appropriate. As will be clear from thegeneral process discussion below, remote plasma-generated excitedspecies facilitate high quality layers as well as possible greater waferthroughput.

Process Flow

FIG. 3 shows a general process sequence in accordance with theinvention, illustrated in the context of forming a transistor gate stackon a germanium surface. Initially a germanium surface is provided 100.The germanium surface can comprise, among other things, an epitaxialgermanium layer, a high [Ge] germanium alloy (e.g., SiGe with [Ge]>20atomic %, particularly for alloys with >40 atomic %) or the top surfaceof a monolithic germanium wafer. The embodiments described herein areparticularly useful for substantially pure (preferably greater than 90%and more preferably greater than 95% pure) germanium surfaces.

Prior to the pretreatment process 110, the germanium layer can beoptionally first cleaned to remove contaminants and naturally occurringor native oxide. Conventionally, wafer cleaning prior to gate oxidegrowth is conducted ex situ prior to loading the wafer into the processchamber. For example, wafers may be cleaned in an SCl/HF wet etch bath.Alternatively, an integrated HF and acetic acid vapor clean can beconducted in a neighboring module within a cluster tool, reducingtransport time and opportunity for recontamination or reoxidation. Forsome applications, the cleaning oxide left by the SCl step is notremoved, but is instead used as the initial oxide layer. In anotherpossibility, a hydrogen bake step can be conducted within the depositionchamber to sublimate native oxide. Small amounts of HCl vapor can beadded to this step to aid in cleaning metal contaminants and the likeduring the hydrogen bake. In still another arrangement, plasma productscan assist or conduct in situ cleaning, such as by substituting Hradicals for hydrogen gas. Due to the low melting point of germanium,however, if cleaning is employed then preferably an ex situ wet clean isemployed.

The wafer or other substrate supporting the germanium surface is loadedinto the process chamber. Even after cleaning, germanium surfaces tendto include inconsistent and undesirable surface characteristics,including poor quality and non-uniform native oxide. Such surfaces canrender subsequent ALD processes, such as the illustrated ALD depositionof ZrO₂ or Al₂O₃, inconsistent in their nucleation, and also causeincorporation of impurities into the structure. GeO and GeO₂ are highlyunstable on germanium surfaces. One method of facilitating consistentnucleation of the subsequent deposition (e.g., adsorption of ALDreactants) is to deposit a thin interfacial layer. Disadvantageously,however, such a layer increases the overall thickness of the dielectricto be formed upon the substrate and also tends to decrease the effectivedielectric constant.

Accordingly, the preferred embodiments employ a surface treatment 110 ofthe germanium surface, preferably conducted in the same chamber as asubsequent deposition 120 of the gate dielectric. The treatment modifiesthe surface termination of the substrate to promote subsequentdeposition. Advantageously, the treatment 110 is tuned such that thereactants and conditions impart sufficient energy to break the surfacebonds (e.g., Ge—Ge bonds) and form new ones, while temperature ismaintained low enough to prevent etching of the substrate or significantdiffusion of active species into the bulk material. Additionally, noappreciable deposition takes place. Preferably no more than a monolayerof terminating tails or bridges is left by the surface treatment 110. Asdiscussed below, while no deposition beyond surface termination takesplace, some conversion (e.g., oxidation, nitridation) of the top fewmonolayers of the substrate can be advantageous.

In general, the germanium surface treatment has a minimal effect on thebulk germanium below the surface. Preferably, the treatment takes placeat less than about 700° C., and more preferably less than about 650° C.Bulk effects to be avoided include deep diffusion of reactants, such asoxygen and nitrogen, into the bulk. Particularly, formation of GeO andGeO₂ in the bulk is to be avoided. These compounds, unlike silicondioxide, are very unstable and in fact GeO is water-soluble. Nitrogenformation in the bulk is less of an issue. Nevertheless, oxidation andnitridation should penetrate less than about 15 Å below the originalsurface of the germanium, more preferably less than about 10 Å, mostpreferably from 2–5 Å. Limiting bulk diffusion avoids detrimentaleffects on the Ge electrically active layer and to inhibits increasingthe dielectric constant or thickening of the equivalent oxide thickness(EOT). Process parameters are thus set to avoid significant oxygenand/or nitrogen diffusion into the bulk, beyond the oxygen and/ornitrogen incorporation in the upper few monolayers of the substrate.Preferably, the bulk germanium (e.g., at a depth of 30 Å or greater)contains less than about 1% atomic concentration of nitrogen and lessthan about 1% atomic concentration of oxygen. At a depth of 10 Å orgreater from the original germanium surface, each of oxygenconcentration and nitrogen concentration is preferably less than about10 atomic %. Preferably, the treatment 110 leaves a surface terminationof oxygen bridges, nitrogen bridges, —NH surface groups, —NH₂ surfacegroups, —OH surface groups, or mixtures of the foregoing.

In accordance with one embodiment, the treatment comprises a plasmatreatment. Preferably, the plasma is generated by a remote plasma, suchthat highly energetic ions do not bombard and damage the germaniumsurface. Reactants include oxygen and/or nitrogen excited species thatare remotely generated and therefore optimized for neutral excitedspecies generation, particularly N and O. Exemplary sources for thenitrogen and oxygen excited species include, but are not limited to,NH₃, O₂, O₃, H₂O, H₂/N₂, H₂N₂, H₂/N₂, N₂O, NO, N₂, carbon-containingreactants such as CO₂, and organic compounds such as acetic acid.Mixtures of the above compounds are also contemplated, particularlymixtures of any of the above compounds with oxygen and with hydrogen.Nitrogen, argon, helium, neon, krypton or other inert gas can also flowto aid formation of glow discharge and serve as carrier gas, but thetotal flow rates and partial pressures are preferably arranged to keepreaction chamber pressure below about 10 Torr for maintaining operationof the remote plasma unit. Preferably, the process parameters are tunedto be sufficient for breaking surface bonds without significant bulkmodification. In the illustrated embodiments, wherein subsequentdeposition 120 is conducted in situ by ALD, temperatures are preferablyin the range of room temperature to 700° C., more preferably from 200°C. to 500° C. For the given temperature, pressure, remote plasma power,reaction times and reactant concentrations are adjusted to achieve thedesired surface conditioning.

Preferably, conditions are arranged such that the above-referenceddiffusion preferences are maintained, while at the same time ensuringfull reaction of the germanium surface to leave oxygen bridges, nitrogenbridges, —NH surface groups, —NH₂ surface groups, —OH surface groups, ormixtures of the foregoing. Accordingly, the substrate can be maintainedanywhere from room temperature up to 700° C. Preferably, a temperatureof the substrate is kept below about 500° C., and more preferably below300° C., to minimize risk of diffusion. An exemplary power level for aremote plasma generator is in the range of 500 W to 2 kW.

In another embodiment, the germanium surface treatment 110 comprises athermal treatment to achieve the same goals of oxygen bridges, nitrogenbridges, —NH surface groups, —NH₂ surface groups, —OH surface groups, ormixtures of the foregoing with minimal oxidation or nitridation into thebulk. Accordingly, the same list of reactants noted above for the remoteplasma reactions can be employed for thermal reactions, as long assufficient energy is provided to form the desired surface termination.Again, depending on reactivity of the reactants, thermal reactions cantake place from room temperature to 700° C., although temperatures arepreferably kept below about 500° C. and more preferably below about 300°C. to avoid diffusion into the bulk.

The germanium surface treatment 110 is most preferably conducted in situin the same chamber as a subsequent dielectric deposition 120. In thiscase, the substrate temperature is preferably set to match that desiredfor the subsequent deposition 120 within the same chamber. Surfacetreatment 110 and deposition 120 can be considered isothermal, withinthe meaning of the present description, if the processes are conductedwithin about 50° C. of one another, more preferably within about 25° C.of one another, and most preferably the target or set point temperaturesare identical so that no temperature ramping is required between steps.

As noted, the process does not result in a deposited layer. Processparameters are preferably selected to replace Ge—Ge bonds in the upperfew monolayers of the substrate with Ge—O bonds, Ge—N bonds —OH surfacegroups and/or —NH surface groups, particularly converting less thanabout 15 Å of the substrate surface to germanium oxide, germaniumnitride or germanium oxynitride. More preferably, the surface treatmentforms less than about 10 < and most preferably about 2 Å to 5 Å onaverage.

Following the germanium surface treatment 110, the gate dielectric isdeposited 120 over the treated surface. The deposition 120 can alsoinclude excited species flow; however, in such a case, the radicalsupply from the prior germanium surface treatment 110 will typically bedifferent from the supply employed in the deposition 120. Furthermore,for the preferred ALD processes, the reaction chamber should be emptied(e.g., purged) of any reactants prior to each reactant pulse.Accordingly, the flow of stable gases or excited species during thegermanium surface treatment 110 is preferably stopped prior todeposition 120.

In accordance with the preferred embodiment, the deposition 120comprises an ALD-type deposition, wherein alternated pulses saturate thesurface, and each cycle leaves no more than about 1 monolayer of thedielectric material. In the examples below, an aluminum source gas orzirconium source gas is alternated with an oxygen source gas to formaluminum oxide (Al₂O₃) and zirconium oxide (ZrO₂). The skilled artisanwill appreciate that similar recipes can be employed to form other highk materials, such as hafnium oxide (HfO₂), tantalum oxide (Ta₂O₅),barium strontium titanate (BST) or strontium bismuth tantalate (SBT).

The first pulse of the ALD deposition advantageously reacts with thetermination left by the germanium surface treatment 110. Alternatively,a further surface treatment can be provided prior to the deposition. Forexample, a water treatment can more readily react with the surface afterthe germanium surface treatment 110, to leave a hydroxyl-terminatedsurface that readily react with subsequent ALD processes.

In one embodiment, either before or after the germanium surfacetreatment 110, the substrate supporting the germanium surface is loadedinto the reaction space of a Pulsar™ 3000 reactor (commerciallyavailable from ASM America, Inc. of Phoenix, Ariz.), which is designedfor ALD processes. The reaction space is evacuated to vacuum with amechanical vacuum pump. After evacuation the pressure of the reactionspace is adjusted to about 5–10 mbar (absolute) with flowing inert gas(e.g., helium, argon or nitrogen gas) that had a purity of 99.9999%.Then the reaction space is stabilized at 300° C. Alternating vapor phasepulses of (CH₃)₃Al and H₂O, vaporized from external sources, areintroduced into the reaction space and contacted with the substratesurface. The source chemical pulses are separated from each other withflowing nitrogen or other inert gas.

Each pulsing cycle consists of four basic steps:

-   -   (CH₃)₃Al pulse    -   N₂ purge    -   H₂O pulse    -   N₂ purge

An exemplary aluminum oxide deposition cycle is summarized in Table I.

TABLE I Al₂O₃ Temperature Pressure Time Phase Reactant (° C.) (mbar)(sec) pulse 1 TMA 300 5–10 0.2 purge 1 — 300 5–10 1.1 pulse 2 H₂O 3005–10 1.5 purge 2 — 300 5–10 3.0

The number of cycles determines the thickness of the layer. The growthrate of Al₂O₃ from (CH₃)₃Al and H₂O is typically near 0.1 nm/cycle or 1Å/cycle at 300° C., or about 3–4 cycles/monolayer (Al₂O₃ has a bulklattice parameter of about 3 Å). The methyl terminations left by eachTMA pulse reduce the number of available chemisorption sites, such thatless than a full monolayer forms with each pulse. The pulsing cycle isrepeated sufficient times to produce the desired layer thickness.Aluminum oxide can be used as the gate dielectric, or as a thin layerprior to forming another dielectric layer.

In another arrangement, ZrO₂ is deposited by an ALD type process. ZrCl₄vapor is introduced to the reaction chamber and exposed the wafersurface for 1.5 s. This is referred to as pulse A. The reaction chamberwas purged with nitrogen gas for 3.0 s to remove surplus ZrCl₄ andbyproducts from the reaction chamber. This is referred to as purge A.Then water vapor was introduced to the reaction chamber and exposed tothe wafer surface for 3.0 s. This is referred to as pulse B. ResidualH₂O and reaction byproducts were removed by purging the reaction chamberfor 4.0 s. This is referred to as purge B. During each of the reactionphases, the reactants are supplied in sufficient quantity for the givenother parameters to saturate the surface.

This exemplary high-k deposition cycle is summarized in Table II.

TABLE II ZrO₂ Temperature Pressure Time Phase Reactant (° C.) (mbar)(sec) pulse A ZrCl₄ 300 5–10 1.5 purge A — 300 5–10 3.0 pulse B H₂O 3005–10 3.0 purge B — 300 5–10 4.0

The cycle of Table II, consisting of pulse A, purge A, pulse B, purge B,was repeated 51 times. The average deposition rate is about 0.59 Å/cycleat 300° C., such that the ZrO₂ thickness was about 30 Å.

More generally, temperatures during an ALD process preferably fallbetween about 200° C. and 500° C., depending upon the acceptable levelof chlorine incorporation into the layer. At higher temperatures, thechlorine content goes down. Too much chlorine can lead to chargetrapping. At 300° C. chlorine content has been measured at about 0.5%.For an amorphous ZrO₂ layer, the temperature is more preferably at thelow end of this range, between about 200° C. and 250° C., and mostpreferably at about 225° C. For a crystalline film, the temperature ismore preferably at the high end of this range, between about 250° C. and500° C., and most preferably about 300° C. As will be appreciated by theskilled artisan, however, mixtures of amorphous and crystallinecomposition result at the boundary of these two regimes. The illustratedprocess produces a largely crystalline ZrO₂ film.

In this case, the metal monolayer formed in the metal phase isself-terminated with chloride, which does not readily react with excessZrCl₄ under the preferred conditions. The preferred oxygen source gas,however, reacts with or adsorbs upon the chloride-terminated surfaceduring the oxygen phase in a ligand-exchange reaction limited by thesupply of zirconium chloride complexes previously adsorbed. Moreover,oxidation leaves a hydroxyl and oxygen bridge termination that does notfurther react with excess oxidant in the saturative phase.

Preferably, sufficient cycles are conducted to grow between about 20 Åand 60 Å of ZrO₂. More preferably, sufficient cycles are conducted togrow between about 20 Å and 40 Å. The dielectric constant of the layeris between about 18 and 24. In the illustrated examples, 30 Å of Zr₂O₃was formed.

Gate dielectric deposition 120 may include multiple deposition steps.However, due to the prior surface treatment, the need for an interfacelayer is reduced, such that a high k material (having a k value greaterthan that of silicon nitride, more preferably greater than 10) ispreferably directly deposited upon the treated germanium surface.Multiple “nanolaminate” sublayers can still have an advantage intailoring crystal structure and/or maximizing dielectric constantwithout sacrificing barrier properties.

Following gate dielectric formation, the gate electrode is deposited 130over the gate dielectric. Once the gate stack has been completed, thegate electrodes are preferably patterned by conventionalphotolithographic techniques and etching. In other arrangements, thegate electrodes can be patterned prior to or after deposition of anoverlying metal layer, and the metal can be employed in a self-alignedsilicidation, as is known in the art. In some arrangements, the gateelectrode itself comprises a metal with a work function tailored to theunderlying germanium.

Having completed the gate stack, further processing to complete theintegrated circuit follows. For example, gate stacks typically areinsulated by blanket deposition of a dielectric and spacer etch.Transistor active areas are then doped to form source and drain regionsto either side of the patterned electrodes, and wiring or “back end”processes complete the circuit.

Advantageously, the germanium surface treatment 110 facilitatessubsequent deposition over the treated germanium surface. In theillustrated example, the germanium surface treatment 110 facilitatesadsorption of ALD reactants.

FIGS. 4A and 4B illustrate a transistor gate incorporating such adielectric stack. In particular, a germanium structure 200 is shown witha transistor gate stack 210 formed over the germanium structure. In theillustrated embodiment, the germanium structure 200 comprises an upperportion of a single-crystal germanium wafer, though the skilled artisanwill appreciate that the substrate can also comprise epitaxiallydeposited germanium or SiGe with a high [Ge].

The gate stack 210 includes an electrode layer 220, with sidewallspacers 230 and an insulating layer 240 protecting and isolating theelectrode 220 in a conventional manner. Also illustrated is a morehighly conductive strapping layer 250, typically including metal, overthe gate electrode 220. The strap 250 facilitates rapid signalpropagation among transistor gates across the wafer, connecting thegates to logic circuits. The gate electrode may comprise a conventionaldoped polysilicon layer, SiGe alloy, or metal alloy with compositionstailored for a desired work function.

A gate dielectric 260, formed by the exemplary processes describedabove, separates the gate electrode 220 from the germanium structure200. As noted in the Background section above, the gate dielectric 260is a critical feature in the pursuit of denser and faster circuits.

As best seen from the enlarged view of FIG. 4B, the preferred gatedielectric 260 includes an interface 262 with the underlying germaniumstructure 200 and a bulk dielectric layer 264. The interface 262 of theillustrated embodiment does not represent a deposited layer; rather, theinterface has been modified by very slight oxidation, nitridation and/orsurface group formation prior to the deposition of the bulk dielectriclayer 264 thereover. In other arrangements, the germanium surfacetreatments could leave additional layers. In the illustratedembodiments, excited species from a remote plasma or thermal reactionsform the interface 262 immediately prior to ALD, where surfacemodification facilitates adsorption of ALD reactants. In the case ofremote plasma nitridation and/or oxidation, the interface 262 comprisesa nitridized and/or oxidized portion of the germanium structure 200preferably extending less than about 15 Å, more preferably less thanabout 10 Å into the germanium structure 200, most preferably comprisingabout 2 Å to 5 Å. Preferably the bulk germanium underneath thisinterface 262 contains less than about 10 atomic % nitrogen and lessthan about 10 atomic % oxygen at a depth of 10 Å or greater depth.

Due to the consistent nucleation (adsorption) offered by the germaniumsurface treatment, in combination with the inherently self-limitingnature of ALD, the dielectric layer 260 can be made extremely thin andyet still exhibit excellent smoothness. The poor surface quality naturalto germanium, and consequently inconsistent deposition on top of thegermanium, is thus overcome. Preferably, the high k layer 260 has athickness of less than about 100 Å, more preferably less than about 50Å, and a surface roughness of less than about 5 Å rms, more preferablyless than about 3 Å rms, and with the truly self-limiting surfacereactions of ALD, even surface roughness of less than 1.5 Å is possible.

It will be appreciated by those skilled in the art that variousomissions, additions and modifications may be made to the processesdescribed above without departing from the scope of the invention, andall such modifications and changes are intended to fall within the scopeof the invention, as defined by the appended claims. For example, whileillustrated in the context of surface treatment prior to ALD and CVD,the skilled artisan may also find application for treatment of germaniumsurfaces prior to other forms of deposition, including but not limitedto MOCVD and JVD.

1. A method of depositing a film over a germanium surface in fabricatingan integrated circuit, the germanium surface having greater than about40 atomic % germanium content, the method comprising: exposing thegermanium surface to an oxygen- and/or nitrogen-containing vapor,thereby forming a modified surface; and atomic layer depositing adielectric material over the modified surface.
 2. The method of claim 1,wherein the oxygen- and/or nitrogen-containing vapor comprises excitedspecies from a remote plasma generator.
 3. The method of claim 1,wherein exposing comprises thermally reacting the oxygen and/ornitrogen-containing vapor with the germanium surface.
 4. The method ofclaim 1, wherein exposing comprises forming less than about 10 atomic %nitrogen and less than about 10 atomic % oxygen at depth of about 10 Åor greater below the germanium surface.
 5. The method of claim 1,wherein the oxygen and/or nitrogen-containing vapor comprises a gasselected from the group consisting of O₂, O₃, H₂O, H₂O₂, NO, N₂O, N₂,N₂/H₂, HCOOH, HClO₃, CO₂, and mixtures of the foregoing.
 6. The methodof claim 1, wherein exposing comprises maintaining the germanium surfaceat a temperature below about 500° C.
 7. The method of claim 6, whereinexposing comprises maintaining the germanium surface at a temperaturebelow about 300° C.
 8. The method of claim 1, wherein the germaniumsurface comprises a single crystal structure having a germanium contentgreater than about 90 atomic %.
 9. The method of claim 1, wherein thegermanium surface comprises a top surface of a silicon germanium layer.10. The method of claim 1, wherein atomic layer depositing comprisesalternating at least one metal precursor with an oxygen precursor. 11.The method of claim 10, wherein the gate dielectric layer ischaracterized by a dielectric constant greater than about
 5. 12. Themethod of claim 11, wherein the gate dielectric layer is characterizedby a dielectric constant greater than about
 10. 13. The method of claim1, wherein exposing and atomic layer depositing are performed in situ ina single deposition chamber.
 14. The method of claim 1, wherein exposingdoes not deposit a layer greater than about one atomic monolayer.
 15. Amethod of depositing on a germanium surface, the method comprising:providing a germanium surface having greater than about 40 atomic %germanium content; forming a surface termination across the germaniumsurface, the surface termination selected from the group consisting ofoxygen bridges, nitrogen bridges, —OH groups, —NH groups, —NH2 andmixtures of the foregoing; vapor depositing a layer directly over thesurface termination.
 16. The method of claim 15, wherein vapordepositing comprises atomic layer deposition.
 17. The method of claim16, wherein the layer comprises a dielectric material having adielectric constant greater than that of silicon nitride.
 18. The methodof claim 17, wherein the layer comprises a dielectric material selectedfrom the group consisting of aluminum oxide, zirconium oxide, hafniumoxide, tantalum oxide and ternary oxides.
 19. The method of claim 15,wherein forming the surface termination comprises providing excitedspecies from a remote plasma generator.
 20. The method of claim 15,wherein forming the surface termination comprises thermal reaction ofthe germanium substrate with vapor reactants at less than about 500° C.21. The method of claim 15, wherein forming the surface terminationcomprises diffusing nitrogen and oxygen to depth of no more than 15 Åbelow the germanium surface.
 22. The method of claim 21, wherein formingthe surface termination comprises leaving less than about 10 atomic %nitrogen and less than about 10 atomic % oxygen at a depth of 10 Å orgreater below the surface termination.
 23. The method of claim 15,wherein forming the surface termination comprises nitridizing and/oroxidizing a top 2–5 Å of the germanium structure.
 24. A method ofdepositing a film over a germanium surface in fabricating an integratedcircuit, comprising: providing a germanium structure having greater thanabout 40 atomic % germanium content; exposing a surface of the germaniumstructure to a surface treatment for improved nucleation of an atomiclayer deposition reactant; and atomic layer depositing a layer over thetreated surface using the atomic layer deposition reactant.